In a study of small subunit ribosomal RNA (SSU-rRNA) gene sequences in cereal and a grass Phaeosphaeria species, group I introns were found in 9 of 10 P. avenaria f. sp. avenaria(Paa) isolates from oat (Avena sativa L.), 1 of 2 Phaeosphaeria sp. (P-rye) isolates (Sn48-1) from Polish rye (Secale cereale L.), 1 Phaeosphaeria sp. (P-dg) isolate (S-93-48) from dallis grass (Paspalum dilatatum Poir.) and both heterothallic P. a. f. sp. triticea (Pat2) isolates (ATCC26370 and ATCC26377) from foxtail barley (Hordeum jubatum L.). There were no group I introns in wheat- and barley-biotype P. nodorum (PN-w and PN-b), homothallic P. a. f. sp. triticea (Pat1) and P. a. f. sp. triticea (Pat3) from the state of Washington. Based on the reference 16S rDNA nucleotide sequence of Escherichia coli (accession number J01695), the intron-inserted positions of Pav.nS943, Pse.nS943, Ppa.nS1199 and Pho.nS1533 were determined to be at nt943, nt943, nt1199 and nt1533, respectively. The sizes of the introns were 362 bp for Pav.nS943 (from Paa), 363 bp for Pse.nS943 (from P-rye), 460 bp for Pho.nS1533 (from Pat2) and 383 bp for Ppa.nS1199 (P-dg). The intron-inserted position at nt1533 found in SSU-rRNA of Pat2 pathogen was newly discovered. The phylogenetic relationships based on aligned conserved secondary structure component sequences of group I introns showed that three introns from cereal Phaeosphaeria species (Pav.nS943, Pse.nS943 and Pho.nS1533) were likely affiliated with subgroup IC1 introns while Ppa.nS1199 intron from the dallis grass pathogen belonged to subgroup IE3.